Wireless communication has been developed and used as a communication technique for eliminating a wiring operation required in existing wired communication and further realizing mobile communication. For example, an example of the standard for a wireless LAN (Local Area Network) is IEEE (The Institute of Electrical and Electronics Engineers) 802.11. IEEE 802.11a/g has already been widespread.
The IEEE 802.11a/g standard supports a modulation method that realizes a maximum communication speed (the physical layer data rate) of 54 Mbps using Orthogonal Frequency Division Multiplexing (OFDM) in the 2.4 GHz band or 5 GHz band. In addition, in the IEEE 802.11n standard, which is an extension of the IEEE 802.11a/g standard, a higher bit rate is realized by employing the MIMO (Multi-Input Multi-Output) communication scheme. The MIMO communication scheme is a communication scheme (an existing communication scheme) that realizes a spatially multiplexed stream using a transmitter and a receiver each including a plurality of antennas. IEEE 802.11n can provide a high throughput that is higher than 100 Mbps. However, with an increase in the amount of information of transmitted content, a higher bit rate is required.
For example, by increasing the number of antennas of the MIMO communication devices and increasing the number of spatially multiplexed streams, the throughput of pier-to-pier communication can be increased while maintaining downward compatibility. However, in the future, the throughput of communication among a plurality of users needs to be increased in addition to increasing the throughput per user in communication.
The working group of IEEE 802.11ac attempts to establish a wireless LAN standard that uses a frequency band lower than or equal to 6 GHz and that realizes a data transmission speed that is higher than 1 Gbps. In order to realize such a wireless LAN standard, a space division multiple access scheme in which a plurality of users share wireless resource on the spatial axis, such as multi-user MIMO (MU-MIMO) or SDMA (Space Division Multiple Access), is a promising scheme.
Currently, space division multiple access is developed as one of base technologies for a next-generation cell phone system based on Time Division Multiple Access (TDMA), such as PHS (Personal Handyphone System) or LTE (Long Term Evolution). In addition, in the wireless LAN technical field, one-to-many communication garners much attention, as described above. However, few applications are available in this field. One of the reasons for that is that it is difficult to efficiently multiplex a plurality of users in packet communication.
Note that a communication system has been developed by using an RTS packet, a CTS packet, and an ACK packet that have a packet format having downward compatibility with IEEE 802.11 and combining the following two techniques: carrier sense of the existing IEEE 802.11 standard and space division multiple access using an adaptive array antenna (refer to, for example, PTL 1).
When the space division multiple access scheme is applied to a wireless LAN, a variable length frame may be multiplexed on the same time axis. At that time, if the lengths of data items transmitted to all of the plurality of users are the same, no problem arises. However, if the lengths of all frames to be multiplexed are not the same due to a difference among the lengths of transmitted data, the level of frame multiplexing during a transmission interval is decreased or increased and, therefore, the total transmission power is abruptly changed. If frames having different lengths are directly multiplexed and transmitted, the received power is abruptly changed on the receiver side due to an increase or a decrease in the level of frame multiplexing. Thus, an unstable operation occurs in terms of auto gain control (AGC). In this way, a variety of problems may arise (e.g., the power distribution in a frame in terms of an RCPI (Received Channel Power Indicator) defined in IEEE 802.11 varies). Accordingly, even when the lengths of data items transmitted to the users are not the same, the frames multiplexed on the same time axis need to be finally transmitted while keeping the same frame length.
For example, in the systems having a fixed frame format (such as an existing cellar system), a frame, for example, can be padded by inserting diversity data (refer to, for example, PTL 2), scheduling assigned times (refer to, for example, PTL 3), using a variable data rate (refer to, for example, PTL 4 or 5), or using a variable channel configuration (refer to, for example, PTL 6). However, since the configuration of such a system radically differs from the configuration of a system using a variable length frame format, such as a wireless LAN system, it is difficult to apply such existing technologies to the system using a variable length frame format.